I have an 05 Sable with the Duratec motor which started to hesitate and stumble every once in a while a couple of months ago. But, since it did it so seldom, that is not every time I would drive it, I didn't think much of it until it started to do it more often. Now, it stumbles for less than a second, but is doing it every time it is driven at least once every minute or so. The car starts and runs just fine otherwise. No codes are present and I check it everyday. Changed the fuel filter a week ago but no luck - still stumbling. Just added some injection cleaner today so I won't know if this has helped for a few days (retired and don't drive much in the snow if I don't have to). The car has just over 47,000 miles on it and has always run extremely well. All maintenance has been done on a regular basis and only full synthetic lubricants have been employed. Any ideas?